The invention relates to holography and, more particularly, to holographic data storage.
Many different types of data storage media have been developed to store information. Traditional data storage media, for instance, include magnetic media, optical media, and mechanical media to name a few. Increasing data storage density is a paramount goal in the development of new or improved types of data storage media.
In traditional media, individual bits are stored as distinct mechanical, optical, or magnetic changes on the surface of the media. For this reason, medium surface area generally poses physical limits on data densities of traditional media.
Holographic data storage media can offer higher storage densities than traditional media. In a holographic medium, data is stored throughout the volume of the medium rather than the medium surface. Moreover, data can be superimposed within the same medium volume using multiplexing techniques. For these reasons, theoretical holographic storage densities can approach tens of terabits per cubic centimeter.
In holographic data storage media, entire pages of information, e.g., bit maps, can be stored as optical interference patterns within a photosensitive optical material. This is done by intersecting two coherent laser beams within the optical material. The first laser beam, called the object beam, contains the information to be stored; and the second, called the reference beam, interferes with the object beam to create an interference pattern that can be stored in the holographic recording material as a hologram. In most conventional holographic recording systems, the object beam and reference beam ordinarily follow separate optical paths.
When a stored hologram is illuminated with only the reference beam, some of the reference beam light is diffracted by the hologram interference pattern. Moreover, the diffracted light can be directed to reconstruct the original object beam. Thus, by illuminating a recorded hologram with the reference beam, the data encoded in the object beam can be reconstructed and detected by a data detector such as a camera.
Self-referenced holography as described in U.S. patent application Ser. No. 09/813,066, filed Mar. 20, 2001 for Jathan Edwards entitled xe2x80x9cSelf-Referenced Holographic Storage,xe2x80x9d can improve holographic data storage systems. In self-referenced holography, the object beam and reference beam follow a common optical path. In particular, in self-referenced holography, the reference beam is created from a zero frequency Fourier component of the object beam. As described in U.S. patent application Ser. No. 09/813,066, the creation of the reference beam from the zero frequency Fourier component of the object beam can be achieved using a lens or a mirror. The entire content of U.S. patent application Ser. No. 09/813,066 is incorporated herein by reference.
For example, a lens can be positioned in the optical path of the object beam before the medium to optically direct the zero frequency component of the object beam in order to create a reference beam. Alternatively, a mirror can be positioned in the optical path of the object beam after the medium to optically reflect the zero frequency component of the object beam after it passes through the medium. Generally, self-referenced holography can yield a number of advantages in a holographic data storage system, including the realization of a single optical path directed toward a holographic recording medium which can reduce the size of the system and possibly reduce alignment concerns.
In general, the invention is directed to self-referenced holographic recording techniques that make use of a diffusive element. In particular, the diffusive element can be positioned in a holographic recording system to create a reference beam from a zero frequency Fourier component of a data encoded object beam. The diffusive element may randomly diffuse the zero frequency Fourier component, or may diffuse the zero frequency Fourier component in an ordered or partially ordered manner. For example, the diffusive element may be designed to create a reference beam having particular angles, phase and amplitude. Such techniques may improve storage capacity of a holographic medium by improving multiplexing of stored holograms in the medium. Multiplexing refers to techniques used to store substantially overlapping holograms in the volume of the holographic recording medium.
In one embodiment, the invention provides a method comprising optically directing a zero frequency Fourier component of a data encoded object beam using a diffusive element to create a reference beam, and illuminating a holographic medium with the reference beam and non-zero frequency Fourier components of the data encoded object beam. In particular, the zero frequency Fourier component of the data encoded object beam, i.e., the reference beam, can interfere with higher order Fourier components of the data encoded object beam to create a hologram in the medium.
In another embodiment, the invention provides a holographic data storage system comprising a holographic medium, and a diffusive element positioned to create a reference beam from a zero frequency Fourier component of a data encoded object beam, wherein the reference beam and non-zero frequency Fourier components of the object beam interfere in the holographic medium to create a hologram.
In an added embodiment, the invention provides a method comprising optically directing a zero frequency Fourier component of the first optical beam using a phase mask to create a second optical beam, and illuminating a holographic medium with the second optical, beam and non-zero frequency Fourier components of the first optical beam.
The different embodiments may provide one or more advantages. In particular, self-referenced holography can realize a holographic system having a single optical path. In addition, removing the zero frequency Fourier component from an object beam to create a reference beam can reduce undesirable intensity of the object beam. In other words, removing the zero frequency Fourier component of the object beam can reduce or avoid undesirable overexposure of photosensitive holographic recording material during holographic recording by reducing the total energy transmitted to the recording material via the object beam.
In addition, use of a diffusive element in self-referenced holography can further improve the holographic system. The diffusive element may create a reference beam having controlled characteristics, such as phase, angles, and intensity. For example, use of a diffusive element in the form of a phase mask can realize a reference beam having phase content that improves the ability to multiplex data stored in the holographic recording medium. Multiplexing refers to a process of recording two or more holograms in the same volume of the medium, e.g., in an overlapping or partially overlapping manner. By using a diffusive element to create a reference beam with particular angles, phase and amplitude, shift-multiplexing can be employed in order to increase the storage capacity of holographic recording media and simplify the mechanical addressing of individual holograms stored by the holographic recording system. In particular, the ability to reconstruct shift-multiplexed holograms with higher selectivity, i.e., higher storage density, may be enabled by a reference beam created to have defined characteristics of phase, angles, amplitude, and the like.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.